Method for application of solid lubricant coatings



United States Patent 3,497,376 METHOD FOR APPLICATION OF SOLID LUBRICANT COATINGS Ladislav E. Wieser, Dayton, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force No Drawing. Filed Oct. 10, 1966, Ser. No. 586,318 Int. Cl. G03g 13/00; B05b 5/02; B44d 1/14 US. Cl. 11717 7 Claims ABSTRACT OF THE DISCLOSURE A method for the application of solid film type lubricants to a metal substrate. The method comprises the following steps. First, a base coat consisting of a mixture of particles of solid lubricant and resinous or vitreous enamel type binder is applied to the metal substrate by mechanical means and sintered. Then, a second or top coat is applied to the base coat by electrostatic means and sintered. The top coat consists of the same or a similar mixture of lubricant and binder particles as the base coat except that the particles of the top coat are smaller in size than the particles of the base coat.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The present invention relates to the application of lubricants of the solid film type to metal and other substrates and surfaces where there is a need for reduction of friction and of abrasive wear.

In the mechanical and particularly in the bearing arts, the higher speeds and loadings and the more severe environmental demands that are constantly being placed upon lubricated components have led to the development of a variety of so-called solid film lubricants. These comprise friction-reducing materials which can be relatively permanently applied to the desired points or surfaces to be lubricated and will there have such dimensional stability as to resist those environmental factors which would tend to remove or destroy the more conventional lubricating oils and greases.

In the advancing technology of high speed and high altitude aircraft, rocket engines, aerospace vehicles and the like, for example, the need for lubricating materials capable of maintaining their position as well as their lubricious characteristics in high vacuum environments particularly at elevated temperatures has been and continues to become more acute and critical. Under these conditions, conventional mineral oils and greases have been found to evaporate rapidly or to degrade to the point that the parts to which they have been applied are without lubrication even at the early and usually quite critical stages of their operation. Consequently, considerable attention has been focused upon solid film lubricants which, because they are in a solid state and relatively permanently adhered to the parts to be lubricated, will remain at their desired location, will not evaporate and will resist degradation in high temperature, high vacuum and high pressure environments. On the other hand, it is not sur prising that the solidity and permanency of such films have usually resulted in loss of lubricity and reductions in wear life to the extent that solid lubricants have not achieved their ultimate potential.

The solid film lubricants of the prior art have consisted generally of resinous or vitreous enamel type binders such as porcelain enamel or silica interspersed within or admixed with which have been a variety of solid or semisolid lubricity-imparting ingredients such as graphite, par- ICC ticularly pyrolytic graphite, mica and the sulfides of lead, tungsten, molybdenum and similar metals. The binder serves to bond the composite film to the bearings or other surfaces to be lubricated as well as to confine and integrate the lubricating element which is usually in the form of discrete and extremely minute dry powdery particles. It is the common practice to premix the binder and lubricating ingredients as dry powder and then to form a slurry, dispersion or suspension thereof so that they may be applied to the bearing surfaces by such mechanical means as brushing, spraying, immersing or the like. In many cases it is also the practice, after the lubricant-binder composition has been thus applied, to subject it to elevated temperatures for the purposes of curing or sintering the film in situ. In other applications however, the binder and lubricant components are co-deposited upon the surface to be lubricated by various plating, electrostatic or electro lytic means.

While these prior art expedients have made possible the application of a solid film lubricant which will hold its position upon exposure of the lubricated parts to high temperature, high vacuum or high pressure conditions, the films have been characterized by very short wear life and by friction coefficients far in excess of those ultimately desired and of these which are achievable by the use of modern oils and greases where these more conventional lubricants can be employed. It has been found moreover that the prior art solid lubricant films, notwithstanding the sometimes delicate and usually complicated procedures by which they are formed and applied, lack uniformity in their structure and density as well as in their bond to the substrate.

It is accordingly an object of this invention to provide an improved method for the application of solid lubricants to a variety of bearing materials and other substrates.

Still another object of the invention is to provide such a method which will improve the wear life and lubricity of the solid lubricant being thus applied.

Yet another object of the invention is to provide such a method which will enhance the uniformity of the structure and density of the solid lubricant film and the strength, uniformity and permanency of its bond to the substrate surface.

Still another object of the invention is to provide a reasonably simple and economical method for the application of uniform, high density solid lubricant films to a variety of substrates in such a manner that the films will continue to supply lubrication at the points of their application over prolonged periods of exposure to high pressure, vacuum, temperature, speed and load conditions.

Still a further object of the invention is to provide a metal surface with a solid lubricant having excellent lubricity and anti-seizing properties combined with a very low friction coefficient in sliding motion.

To achieve these and other objects and advantages which will appear from a reading of the following disclosure, the present invention provides a two-step coating and sintering technique wherein a first base coat of the lubricant-binder composition is applied to the substrate and sintered and a second or top coat is thereafter applied to the base coat and is itself then sintered. In a preferred embodiment of the method of this invention, the first or base coat is applied by conventional mechanical means such as painting, spraying, or dipping; while the second or top coat is applied to the base coat by electrostatic means such as electrostatic spraying or electrophoretic deposition. In a further refinement of this particular embodiment, the sizes of the lubricant and binder particles forming the top coat are considerably smaller than the sizes of the particles of the same materials forming the base coat. In still another modification of the method of this invention, the top coat, once it has been applied and sintered upon the component to be lubricated, is subjected to a polishing operation, for example, by light abrasive contact with a quartz or polished steel plate.

It is theorized that the extremely fine particles or powder of the binder and lubricating components in the top film coating tend to fill voids which are present in the base coating, often as a result of the driving off of volatiles in the base coat during its sintering stage. Where the top coating is applied by a method which allows these voids to be filled with more of the binder-lubricant mixture, it has been found that, after the second sintering, the composite two-layer film is characterized by superior anti-friction and wear capabilities as well as improved film density.

The invention thus generally described may be more clearly understood by reference to the following detailed descriptions of certain preferred embodiments and examples thereof. A typical process for applying a double layer solid lubricant film according to the present invention comprises the preparation of the substrate or surface to be lubricated which may be a hardened or unhardened steel or steel alloy or any of a variety of other metals or structural materials which may or may not contain platings or surface coatings of copper, gold, silver, titanium or other metals. In the case of a conventional steel alloy, one such surface preparation comprises lightly sand blasting the surface to be lubricated and cleaning it with a solvent such as acetone. In certain instances this preparation may also include oxidizing the surface to be lubricated for example by exposing it to the atmosphere for a period of time on the order of ten minutes at elevated temperatures of from 800 to 1000 degrees Fahrenheit.

The binder in one preferred embodiment of the invention is formed from a frit which is itself composed of a variety of conventional materials which are adapted to be fused into a vitreous enamel at a sintering temperature which is consistent with the temperature that can be withstood by the substrate upon which the film is ultimately to be applied. The binder forming frit may be composed of one or a mixture of such vitreous materials as lead oxide, silicon dioxide, lithium oxide, sodium oxide, potassium oxide, titanum dioxide, lead trioxide, boron trioxide, zinc dioxide, cadmium oxide or sodium silica fluoride. The lubricant particles to be amixed with the binder powder formed by pulverizing the frit may be one or more of the several typical solid or semi-solid dry lubricant materials such as tungsten disulfide, molybdenum disulfide, boron nitride, iron oxide, silver sulfate, ferrous chloride, lead oxide, ferrous sulfide, silver, graphite and mica. According to conventional practice, the binder and lubricant particles in dry form are admixed with each other and with appropriate additives for improving the bond, tear strength, Water resistance and related properties of the resultant film. The powdered ingredients are then placed in a solution, suspension or dispersion such as aqueous solution having sufiicient liquidity to permit spraying, painting or dipping.

In one specific method for practicing the invention, the inorganic binder and the proper proportion of lubricant solids are dispersed in distilled water, are thoroughly mixed by stirring and are then sprayed with a fine air brush under fifty-five pounds per square inch of pressure on the bearing surface which has been cleaned and prepared according to the above described procedure. As a further preparation the surface may be preheated to 150 degrees Fahrenheit or thereabouts before the base coat of the solid lubricant film is applied. The spraying may involve two or three steps to build up a base coat of from .0008 to .0012 inch in thickness. The sprayed specimen may then be placed in an electric oven with dry nitrogen gas atmosphere and sintered at temperatures between 1400 and 1450 degrees Fahrenheit until the composition is fused or sintered. In a particular sintering operation, the coating may be subjected to the stated temperature range in s ccessive five minute intervals for a total of twenty minutes after which the film may be cooled under the same dry nitrogen atmosphere down to 250 degrees Fahrenheit.

In the application of the second or top layer of the two-coat film, the substrate with the base coating thereon is subjected to electrostatic coating either by electrophoresis, electrostatic spraying, glow discharge bombardment or other electrical means until an additional layer of the same or a similar binder-lubricant composition is built up to a thickness or not more than .001 inch. By way of specific example, where electrophoretic deposition is employed to build up this second coating of the lubricant-binder film, a colloidal or gross dispersion of the binder and lubricant particles may be created and the coated substrate made one of a pair of closely spaced electrodes immersed within the dispersion. The electrodes may then be energized by a current with a potential of from 200 to 1,000 volts and such energy level maintained until the desired thickness of the second coating of the binder lubricant material is deposited upon the substrate electrode. Where the second coating is applied by electrostatic spraying on the other hand, the finely divided or powdered binder and lubricant particles may be agitated, blown through or otherwise suspended in the air or other gaseous medium surrounding the closely spaced electrodes and an electromotive potential difference of on the order of from 20,000 to 40,000 volts established between them, whereupon the second coating of the binderlubricant material will be deposited upon the first-coating of the substrate to accomplish substantially the same effects as those achieved by the electrophoretic deposition.

After the second coating has been thus applied, the substrate with both of the film layers thereon is again placed in an electric oven under a dry nitrogen atmosphere and again sintered by exposure to elevated temperatures ranging between 1 350 and 1400 degrees Fahrenheit in the same manner as was the first coating and is then allowed to cool to at least 250 degrees Fahrenheit while still within the dry nitrogen atmosphere. As a final step, the second or top coating of the lubricant-binder composition thus applied may, after it is cooled, be polished with a quartz or polished steel surface.

A specific solid lubricant composition to be applied according to the method of this invention may be composed of 45.6 weight percent of the molybdenum disulfide, 21.4 weight percent of lead sulfide, 3.6 Weight percent of calcium nitrate and 29.4 weight percent of an inorganic or ceramic binder. The binder or bonding composition thus incorporated in the dry lubricant is itself composed of 36.61 weight percent of silicon dioxide, 20 .63 weight percent of sodium carbonate, 6.56 weight percent of aluminum trioxide, 0.95 weight percent of cobalt trioxide, 0.95 weight percent of cadmium oxide or nickel oxide, 2.34 weight percent of calcium oxide, 3.75 weight percent of lithium nitrate and 28.20 weight percent of In the manufacture of the dry lubricant composition, the ceramic binder ingredients in dry powdered condition and in the weight proportions set forth above are thoroughly mixed in dry form and then placed in a crucible or other heating vessel in a furance pre-heated to 2200 degrees Fahrenheit and are there maintained until the mixture becomes molten and colorless, whereupon it is poured into a steel beaker containing cold distilled Water to form a light and frangible frit which is then placed in a drying oven at 250 degrees Fahrenheit for 16 hours whereupon it is then milled or otherwise broken into a fine powder passing a No. 325 mesh sieve. The binder powder thus formed is then admixed with the other proportions of the solid lubricant material set forth above to obtain the dry lubricant material which is then admixed with a suitable amount of water such as from one to three volumes of water to supply a sprayable mixture. For the purposes of testing, this mixture is applied by spraying to a stainless steel or tool steel ring with a 1.375 inch diameter wherein the rings have been first sandblasted and oxidized at 800 degrees Fahrenheit for ten minutes before the lubricant composition is applied in a first coat by spraying. The specimens are then sintered in dry nitrogen gas at from 1400 degrees to 1450 degrees Fahrenheit. The coating thus sintered is then electrostatically sprayed with a second coating of molybdenum disulfide and boron trioxide in a three-to-one weight ratio (these ingredients themselves being in dry powdered form of a particle size to pass a No. 325 mesh sieve), and the second coating is then sintered in dry nitrogen gas at from 1350 to 1400 degrees Fahrenheit.

Where the ring and shoe of a conventional friction and wear test apparatus are composed of RENE No. 41 Steel, the twice-coated, twice-sintered solid lubricant coating is applied as above described and the specimen is loaded at 25 pounds and operated at 600 revolutions per minute in the test rig, the specimens completed over two million revolutions operating in a temperature of 465 degrees Fahrenheit at a partial vacuum of 0000024 pound per square inch, 605,000 revolutions operating at 600 degrees Fahrenheit at .0000014 pound 'per square inch and 558,000 revolutions at 600 degrees Fahrenheit at .0000013 pound per square inch. Where both the ring and shoe are composed of ASTM 4130 Steel, the test apparatus loaded at as high as 52 pounds and operating in a partial vacuum of as low as .0000013 pound per square inch at 600 degrees Fahrenheit operated through 700,000 revolutions at the rate of 600 revolutions per minute.

By way of comparison, the specimen composed of 4130 Steel was sandblasted, cleaned and oxidized at 800 degrees Fahrenheit for ten minutes before receiving a one-coat dry lubricant consisting of molybdenum disulfide and lead sulfide in a two-to-one weight ratio in combination with the above-described ceramic binder. This single coat was applied by an air brush and cured for five minutes at 1300 degrees Fahrenheit in dry nitrogen gas. Where the test specimens were loaded with 25 pounds and operated at the rate of 600 revolutions per minute at a temperature of 600 degrees Fahrenheit the greatest number of revolutions achieved was less than 42,000 where the test took place in a partial vacuum of .0000032 pound per square inch. From these comparable test conditions, it can be seen that the method of the present invention improves the wear life of the lubricated components by on the order of at least fifteen fold. Under varying conditions of load, temperature and pressure, comparison of coatings applied according to the method of the present invention with prior art dry lubricant coatings has demonstrated that the two-coat-doublesintering technique will improve the wear life of the lubricated components for as much as from ten to fifty times.

While the within invention has been described in considerable detail in connection with certain specific examples and embodiments thereof, it is to be understood that the foregoing particularization and detail have been for the purposes of illustration only and do not limit the scope of the invention.

I claim:

1. A method for applying a solid lubricant to a metal substrate, said method comprising applying to said substrate by mechanical means a base coat of a lubricantbinder composition made up of a mixture of particles of binder and particles of solid lubricant, sintering the base coat thus applied, thereafter applying by electrostatic means to the sintered base coat a top coat of a lubricantbinder composition made up of a mixture of particles of binder and particles of solid lubricant, the particles applied in said top coat being at least in part smaller in size than the particles of said base coat, and thereafter sintering said top coat, said binder particles being of a vitreous material comprising at least one of the materials selected from the group consisting of lead oxide, silicon dioxide, lithium oxide, sodium oxide, potassium oxide, titanium dioxide, lead trioxide, boron trioxide, zinc dioxide, cadmium oxide, sodium silica fluoride, aluminum trioxide, cobalt trioxide, nickel oxide, calcium oxide, lithium nitrate, sodium carbonate, and the boron compound having the formula K B O and said lubricant particles comprising at least one of the materials selected from the group consisting of tungsten disulfide, molybdenum disulfide, boron nitride, iron oxide, silver sulfate, ferrous chloride, lead oxide, ferrous sulfide, silver, graphite, mica, lead sulfide, and calcium nitrate.

2. The method according to claim 1 wherein said mechanical means comprises spraying.

3. The method according to claim 1 wherein said electrostatic means comprises electrostatic spraying.

4. The method according to claim 1 wherein said mechanical means comprises painting.

5. The method according to claim 1 wherein said mechanical means comprises brushing.

6. The method according to claim 1 wherein said mechanical means comprises immersing.

7. The method according to claim 1 wherein said electrostatic means comprises electrophoresis.

References Cited UNITED STATES PATENTS 2,251,410 8/1941 Koehring et al. 29-182.2 2,339,392 1/1944 Garner 11722 X 2,571,608 10/1951 Plagge 117-17 X 2,700,623 1/1955 Hall 252-12 2,775,531 12/1956 Montgomery et al. 117-22 2,994,654 8/1961 Fahnoe et al. 11722 X 3,194,702 7/1965 Geller et al. 117 17 3,242,076 3/1966 Hagan 117--22 X 3,336,903 8/1967 Point 117l7 X 3,360,347 12/1967 Todd 29-182.1

WILLIAM D. MARTIN, Primary Examiner US. Cl. X.R. 

